An assistant medical examiner at the Harris County Institute of Forensic Sciences said the child suffered a perforated intestine, "which he believed had leaked bile into his body, causing his death," the arrest warrant states.

Such injuries "typically take 48-72 hours before death results," according to an arrest warrant filed Wednesday.

Mother called for help

The baby's mother called 911 when she found the boy unresponsive at their apartment in the 6000 block of Mapleridge Street. She told investigators that she was in another room and went to her son after hearing him cry. Rodriguez-Escaramuza told her that he had been play boxing with the child and struck him in the abdomen, investigators said.

The mother "was adamant she had not assaulted the child," police said. "(She) did account for some minor abrasions to the child, explaining that had fallen in the playground days before."

Rodriguez-Escaramuza told medical personnel at Texas Children's Hospital that he did not harm the child. But when questioned by police, "he eventually admitted to 'play boxing' with his stepson," the warrant said.

Not just 'play'

The assistant medical examiner told police that the death stemmed from more than play.

"It would not result with the 'play boxing' description provided by Rodriguez," the arrest warrant states.

The couple's two other children - a 2-year-old girl and 11-month-old son - have been placed in the custody of relatives, Olguin said. The mother could face criminal charges, Olguin said. "We're still trying to determine if she was aware of the abuse of her child, or was involved in the abuse," she said.